The 1923 Kanto earthquake occurred along the Sagami trough (central Japan), causing severe damage in the Tokyo metropolitan area. This study was able to characterize the source process for this event using geodetic, teleseismic, and strong‐motion data. The Kanto region is located above a large‐scale sedimentary basin. Therefore, 3D Green’s functions and a curved fault modeling the subduction interface geometry were used to account for 3D complex wave propagation inside the basin. The later phases of the 3D Green’s functions with long paths inside the basin had large amplitude and long duration, primarily because of amplification effects caused by the lateral heterogeneity of sedimentary layers. However, 3D static displacements were large mainly because of amplification effects caused by the presence of thick soft sedimentary layers in the basin. The geodetic inversions with 3D and 1D Green’s functions had smaller seismic moments than half‐space ones. This suggests that wave amplifications caused by large‐scale sedimentary basins exert significant effects on seismic moment determinations. The joint inversion with 3D Green’s functions showed two large slip areas with a maximum slip of >6 m and an estimated total seismic moment of about 4.1×1020 N·m (Mw 7.7). Strong motions were recovered very well, even for the later phases, by slips in shallow areas. Furthermore, by comparing various source processes inverted by two fault models of 70 or 280 subfaults, 3D or 1D Green’s functions, and a curved or planar fault model, we verified these assumptions based on the inversion results.